There have been proposed various methods for efficiently coding and decoding audio signals. Compressive coding methods for audio signals having frequency bands exceeding 20 kHz such as music signals, MPEG audio and Twin VQ (TC-WVQ) have been proposed. In a coding method represented by MPEG audio system, a digital audio signal on a time axis is transformed to data on a frequency axis by using orthogonal transformation such as cosine transformation, and the data on the frequency axis is encoded from acoustically important data by utilizing acoustic characteristics of human beings, while acoustically unimportant data and redundant data are not encoded. On the other hand, Twin VQ (TC-WVQ) is a coding method in which an audio signal is represented with data quantity considerably smaller than that of the original digital signal by using vector quantization. MPEG audio and Twin VQ are described in “ISO/IEC standard IS-11172-3” and “T. Moriya, H. Suga: An 8 Kbits transform coder for noisy channels, Proc. ICASSP 89, pp.196-199”, respectively.
Hereinafter, the outline of the general Twin VQ system will be described with reference to FIG. 10.
An original audio signal 101 is input to an analysis scale decision unit 102 to calculate an analysis scale 112. At the same time, the analysis scale decision unit 102 quantizes the analysis scale 112 to output an analysis scale code sequence 111. Next, a time-to-frequency transformation unit 103 transforms the original audio signal 101 to an original audio signal 104 in frequency domain. Next, a normalization unit (flattening unit) 106 subjects the original audio signal 104 in frequency domain to normalization (flattening) to obtain an audio signal 108 after normalization. This normalization is performed by calculating a frequency outline 105 from the original audio signal 104 and then dividing the original audio signal 104 with the calculated frequency outline 105. Further, the normalization unit 106 quantizes the frequency outline information used for the normalization to output a normalized code sequence 107. Next, a vector quantization unit 109 quantizes the audio signal 108 after normalization to obtain a code sequence 110.
In recent years, there has been proposed a decoder having a structure capable of reproducing an audio signal by using part of code sequences input thereto. This structure is called “scalable structure”, and to encode an audio signal so as to realize the scalable structure is called “scalable coding”.
FIG. 11 shows an example of fixed scalable coding which is employed in a general Twin VQ system.
According to an analysis scale 1314 decided from an original audio signal 1301 by an analysis scale decision unit 1303, an original audio signal 1304 in the frequency domain is obtained by a time-to-frequency conversion unit 1302. A low-band encoder 1305 receives the original audio signal 1304 in the frequency domain and outputs a quantization error 1306 and a low-band code sequence 1311. An intermediate-band encoder 1307 receives the quantization error 1306 and outputs a quantization error 1308 and an intermediate-band code sequence 1312. A high-band encoder 1309 receives the quantization error 1308 and outputs a quantization error 1310 and a high-band code sequence 1313. Each of the low-band, intermediate-band, and high-band encoders comprises a normalization unit and a vector quantization unit, and outputs a low-band, or intermediate band, or high-band code sequence including a quantization error and code sequences output from the normalization unit and the vector quantization unit.
In the conventional fixed scalable coding shown in FIG. 11, since the low-band, intermediate-band, and high-band encoders (quantizers) are fixed, it is difficult to encode the original audio signal so as to minimize the quantization errors against the distribution of the original audio signal as shown in FIG. 12. Therefore, when coding audio signals having various characteristics and distributions, sufficient performance is not exhibited, and high-quality and high-efficiency scalable coding cannot be realized.